Laser gyroscopes have a gas laser which amplifies electromagnetic waves passing around a common path of a ring defined, for example, by reflecting mirrors. The amplification which results from interaction of the waves with excited states of atoms can produce oscillations at one or more frequencies for waves traveling in the clockwise direction around the laser as well as counterclockwise around the laser.
With a two wave or frequency system, it has been found that, for low rates of rotation corresponding to a small theoretical difference frequency, the actual output difference frequency is zero or substantially less than would be expected due to the phenomena known as lock-in. It is believed that the lock-in problem arises because of coupling between the waves which may arise from a number of possible factors including back scattering of laser energy from elements within the laser path such as mirrors or a polarization dispersive structure or from scattering centers within the laser gain medium itself.
The attempts to compensate for this problem included one proposal in which the two beams are biased at zero rotation away from the zero output level by the use of a Faraday rotator which subjects beams propagating in different directions to different delay times. However, simply biasing the two beams sufficiently far apart to avoid lock-in produced such a large frequency difference between the two beams that the change in frequency caused by ordinary amounts of rotation was rather insignificant compared to the total frequency difference. Thus, any small drift could obliterate the actual desired signal output. Further attempts to achieve biasing included one in which the Faraday rotator was switched from one direction to another using a symmetric AC switching waveform. Such systems have proven somewhat difficult to implement since the symmetry of the AC switching waveform had to be maintained to greater than one part in a million.
The most successful laser gyroscopes yet proposed and constructed employ four waves of two pairs or beams each propagating in opposite directions. Such systems are shown and described in U.S. Pat. Nos. 3,741,657 and 3,854,819 to Keimpe Andringa and assigned to the present assignee, the specifications of those patents being herein incorporated by reference. In such a laser system, circular polarization for the four waves is preferred. The pair of waves propagating in the clockwise direction includes both left-hand and right-hand circularly polarized waves as does the pair propagating in the counterclockwise direction.
Two biasing components are provided. A device such as a crystal rotator produces a delay for circularly polarized waves that is different in one sense or handedness of circular polarization than for the opposite sense and is also reciprocal. That is, a wave traveling in either direction through the crystal will be delayed by the same amount of time. Secondly, a device such as a Faraday rotator is also disposed in the wave path. Such a device is nonreciprocal providing a different time delay for the two directions of propagation. This is achieved by rotating the circular polarization vector by different angles. The delay is independent of the sense of polarization. The result of these biasing operations produces four waves, two with frequencies above the peak of the gain curve of the laser medium and two below. The two above may, for example, both be right-hand circularly polarized while the lower two are left-hand circularly polarized. At a zero rate of rotation, the frequency difference between the left-hand circularly polarized and the right-hand circularly polarized waves are equal. When, for example, the system is rotated in one direction the right-hand circularly polarized waves will move closer together in frequency while the left-hand circularly polarized waves will move apart. The opposite direction of rotation produces the opposite direction of change in frequencies. The actual rotation rate is readily related to the difference between the difference in right-hand circularly and left-hand circularly polarized wave pairs.
In the laser gyroscope systems disclosed in the referenced patents, a structure for adjusting the length of the path through which the four waves propagate to maintain the frequency pairs positioned symmetrically about the center maximum gain frequency of the laser gain medium curve is described. Such symmetric positioning is desired in order to minimize residual drift or lock-in effects.
The gain of the waves passing through the lasing medium is normally a fraction of a percent and must be sufficient to overcome losses in the medium of the ring cavity such as reflection losses at the mirrors and at windows of the gas laser. The gain of the laser can be increased by increasing the discharge current. However, discharge oscillations in the range from a few hertz per second, dependent on power supply constants, to many megahertz are encountered. The megahertz discharge oscillations cannot be prevented by power supply design since they are predominantly a function of the discharge path geometry and the internal negative resistance of the laser tube gas discharge. Such oscillations cause variations in laer amplification so that the laser gyroscope output will be unstable and erroneous. As a result, the laser amplifier in laser gyros needed to be relatively large and operated at low current to prevent gas discharge oscillations so that overall gain would be sufficient to overcome the losses in the ring cavity. In addition, the amount of energy which could be extracted from the ring cavity to drive the output circuitry was generally severely limited, due to the minimal amount of laser amplifier gain.